Update README.md

Author: apilayer-admin

README.md

@@ -361,6 +361,7 @@ API | Description | Auth | HTTPS | CORS |
 | [ddownload](https://ddownload.com/api) | File Sharing and Storage | `apiKey` | Yes | Unknown |
 | [Dropbox](https://www.dropbox.com/developers) | File Sharing and Storage | `OAuth` | Yes | Unknown |
 | [File.io](https://www.file.io) | Super simple file sharing, convenient, anonymous and secure | No | Yes | Unknown |
+| [Filestack](https://www.filestack.com) | Filestack File Uploader & File Upload API | `apiKey` | Yes | Unknown |   
 | [GoFile](https://gofile.io/api) | Unlimited size file uploads for free | `apiKey` | Yes | Unknown |
 | [Google Drive](https://developers.google.com/drive/) | File Sharing and Storage | `OAuth` | Yes | Unknown |
 | [Gyazo](https://gyazo.com/api/docs) | Save & Share screen captures instantly | `apiKey` | Yes | Unknown |